The present invention relates to a method of preparing diaphragms of deposited asbestos consolidated by a polymer emulsion, which are used in the electrolysis cells for the production of chlorine and alkali hydroxides.
Numerous methods have been proposed for the preparation of diaphragms of deposited asbestos consolidated by a polymer, and the typical method comprises the step of suspending asbestos fibers and an electrolyte-resistant solid polymer having powder or fiber form in an aqueous suspension medium, the step of depositing the asbestos fibers and the polymer on a metal screen cathode by means of filtration, and the step of heating the depositing layer of asbestos fibers containing the solid polymer at a temperature above the melting point of the polymer. The heating step is required to bind together the asbestos fibers with the melted polymer to result in the consolidation of the diaphragm.
As an improved method of consolidating diaphragms of deposited asbestos with a polymer, a method of employing an emulsion of an electrolyte-resistant polymer is proposed. When the deposited layer of asbestos fibers containing the polymer in a form of dispersed phase is dried to remove the suspension medium, the polymer becomes an adhesive to bind the asbestos fibers. Since the drying step is generally effected at room temperature or a temperature below 100.degree. C., the step is remarkably meritorious when compared with the heating step usually conducted at such high temperatures as 300.degree.-400.degree. C.
In order to achieve an effective consolidation of asbestos diaphragms with a polymer emulsion, it is necessary to make the polymer emulsion disperse homogeneously in the slurry of asbestos fibers and further make the polymer distribute uniformly in the asbestos fibers deposited on a metal screen cathode, since the polymer in the asbestos fibers does not flow during the drying step utterly different from the heating step.
In the commercial operation of the electrolysis, the most widely used suspension medium of asbestos fibers is an aqueous solution of alkali hydroxide. Since a polymer emulsion tends to coagulate in an alkaline water, it is difficult to obtain a homogeneous suspension of asbestos fibers containing a uniformly dispersed polymer emulsion in so far as an alkaline water is used as the suspension medium of asbestos fibers. For overcoming the problem of consolidating the asbestos fibers with a polymer emulsion, a method is proposed in which the deposited asbestos fibers are first washed with water and then the alkali-free asbestos fibers are impregnated with a polymer emulsion. However, the method requires the additional steps of washing with water, and the impregnation.